We couldn't find the letter yesterday because he _ (hide)it in the wardrobe.

We couldn't find the letter yesterday because he _ (hide)it in the wardrobe.
We couldn't find the letter yesterday because he _ (hide)it in the wardrobe.

We couldn't find the letter yesterday because he _ (hide)it in the wardrobe.
这里用过去式hid就行了.

had hid过去完成时表示该动作先发于couldn't find 这个动作